In recent years, a technology which is to expand oil country tubular goods by 10% to 30% and is to use in a well is developed, and the technology has greatly contributed to reduction of development costs of an oil well or a gas well. In general, after the steel pipe is expanded to a circumferential direction, collapse pressure decreases because of bauschinger effect. The bauschinger effect is a phenomenon in which, in case that plastic strain is applied by plastic deformation and subsequently stress is applied to a direction opposite to the direction where the plastic strain is applied, yield stress to the direction opposite to the direction where the plastic strain is applied decreases as compared to before the plastic deformation.
For the problem, Patent Documents 1 and 2 suggest the steel pipes which are excellent in collapse properties after pipe-expanding. The steel pipe disclosed in the Patent Document 1 is that microstructure includes a bainite or a bainitic ferrite and the collapse pressure increases by dislocation pinning of solid-soluted C after pipe-expanding. The steel pipe disclosed in the Patent Document 2 is that microstructure consists of a dual phase structure which is the ferrite and a finely dispersed martensite and which suppresses the bauschinger effect.
In addition, when pipe-expanding ratio becomes large, cracks may initiate locally because thickness of the steel pipe may thin locally. For the problem, Patent Document 3 suggests the steel pipe in which microstructure is controlled to a dual phase structure which has a soft phase and a hard phase, because an increase in a work hardening coefficient (n value) is effective.
In the microstructure of the steel pipe disclosed in the Patent Document 3, the soft phase is the ferrite, a tempered martensite, or a tempered bainite, and the hard phase is a martensite-austenite constituent (MA). The MA is obtained by heating the steel pipe to the two phase region and air-cooling.